1. Field of the Invention
The present invention relates to a shaking control unit for a hydraulic cylinder, wherein a first predetermined pressure is admitted to the piston space of the cylinder through a first hydraulic line.
2. Description of the Prior Art
Hydraulic cylinders are used in underground mining for a variety of purposes. For example, in shield-type support frames, so-called packing or sliding caps are synchronously advanced together with a conveyor hinged to the support frame. The cap is used to uphold the roof which is exposed due to the advancing extraction of coal and to close empty spaces. Since the extraction apparatus, for example, a coal plane, does not extend to the plane of the roof, upper coal or residual coal frequently remains attached to the roof. This residual coal presents an obstacle to the advancement of the sliding cap. The sliding cap may have at its front edge a tearing ledge or loosening wedge by means of which the cap separates the residual coal as it is being advanced with normal pressure. However, this advancement is only possible if the residual coal is not adhering too firmly.
Another field of application of hydraulic cylinders is the compacting of filling material which is introduced into the packing space. It is conceivable to use the hydraulic cylinder to compact the material to a certain extent.
If, in the above-explained example, the residual coal adheres very firmly to the roof of a seam, the pressure acting on the hydraulic cylinder actuating the sliding cap as the sliding cap is being advanced may not be sufficient to separate the residual coal. In these cases, it was necessary in the past to use hand tools. Therefore, it has been proposed that in cases like this the normal advancement of the sliding cap be superimposed by a shaking or vibrating movement of the sliding cap, i.e., an alternating loading and unloading of the sliding cap generated by pressure variations in the hydraulic cylinder. The pulse-like pressure increases are intended to separate the remaining coal from the roof of a seam.
It is, therefore, the primary object of the present invention to provide a shaking control unit for a hydraulic cylinder which creates with simple means pulse-like pressure variations in the piston space of the hydraulic cylinder in order to obtain a shaking effect.